Abstract
The tribological properties of two distinctive alkanethiol SAMs, 16-mercaptohexadecanoic acid (MHA) and 1-octadecanethiol (ODT), on gold substrates in various humidity conditions were examined by lateral force microscopy (LFM). The results suggest that hydrophobic ODT SAM is insensitive to humidity. The difference of lateral force signal is within ±10% regardless of humidity. The lateral force signal of hydrophilic MHA SAMs has a significant decrease in signal in humid environments. The influence of bulk water was also investigated by LFM. By imaging under water, the capillary force is eliminated on ODT SAMs, which leads to a lower lateral force. However, the lateral force image was reversed on MHA SAMs, which suggested that hydrophobic forces dominated in water.
Highlights
Tribology is one of the most important mechanical properties for the interaction of surfaces
The tribological properties of hydrophobic and hydrophilic alkanethiol selfassembled monolayers (SAMs) on gold substrates under various humidity conditions were examined by lateral force microscopy (LFM) in this study
The magnitude of the lateral force signal slightly increases by 10% and 5% at 20% Relative humidity (RH) and 40% RH, respectively, but decreases by 10% at 60% RH
Summary
Tribology is one of the most important mechanical properties for the interaction of surfaces. With the development of micro- and nanotechnology, the need for understanding tribological characteristic in micro- and nanoscale systems is important. The tribological properties, including friction, adhesion, and wear, in MEMS are more critical than in bulk devices [4]. In order to effectively reduce the friction in the contact boundary, lubricants in micro/nanoscale are of significant interest to tribology research. Since the surface separation of a MEMS device is typically less than 5 μm, ultrathin films with high packing density have become popular lubricant materials. It is because of these characteristics that SAMs have the potential of being an ideal boundary protection layer and lubricant in MEMS. Among different types of SAM coating, alkanesilane molecules form rigid chemical bonds and well-ordered monolayers are formed on hydroxyl terminated surfaces and alkanethiol molecules on noble metal substrates
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